The present invention relates to a pulse driven tachometer apparatus particularly applicable to internal combustion engines employing capacitive discharge ignition systems.
Capacitive discharge ignition systems have been successfully employed for firing the spark plugs of internal combustion engines, particularly two-cycle engines. Generally, in capacitive discharge ignition systems, a single firing or ignition capacitor, or a group of alternately operable capacitors, is charged to a selected voltage level. The charging source may be a battery-converter unit or an engine-driven alternator unit. The ignition capacitor is connected in a discharge circuit in series with a pulse transformer and an electronic switch such as a silicon controlled rectifier. When the switch is turned on, the fully charged ignition capacitor rapidly discharges through the pulse transformer for firing the associated spark plug of the engine. Although various systems have been developed, they all generally have a capacitor voltage wave which exhibits a very steep wave front as the capacitor is discharged and the number of capacitor discharges per minute exactly equals the engine revolutions per minute (RPM) multiplied by the number of times the capacitor discharges in one engine revolution. The frequency of the capacitor's discharges is therefor a true and accurate indication of the speed of the engine and can be employed to drive a suitable tachometer circuit for providing an engine speed or RPM indication.
Prior art tachometer circuits which have been satisfactorily employed on engines having capacitive discharge ignition systems generally include a charge transfer or measuring or "bucket" capacitor in series with a direct current meter and a rectifying means. The measuring capacitor is filled or charged with a selected electical charge value in response to each discharge of the ignition capacitor and is then fully discharged or emptied. The value of charge to which the measuring capacitor is filled multiplied by the frequency of such charging constitutes a precisely calibrated average current flow which is directly proportional to the speed of the engine. If such a capacitor is connected in series with a suitable microammeter, the output reading mechanism of the meter can be graduated in revolutions per minute to provide a direct reading thereof. In a typical prior art circuit, the measuring capacitor is connected in series with a diode and a microammeter to the ignition capacitor circuit by a resistor-capacitor coupling network. The opposite sides of the measuring capacitor are connected to ground or to reference potential by suitable diodes, with the diode on the input side of the measuring capacitor being a Zener diode. During the charging of the ignition capacitor, current also flows from the charging source through the coupling network into the measuring capacitor-diode network. The forward drop of the diodes are essentially constant and equal and, consequently, the voltage across the measuring capacitor is essentially zero. During charging of the ignition capacitor, no current passes through the meter as a result of the blocking diode in the meter circuit.
At the instant the ignition capacitor discharges, the voltage changes abruptly and the measuring capacitor is coupled through the resistor-capacitor network to a relatively high transient voltage of an opposite polarity. The diode in the meter circuit is now forward biased and initiates current flow through the measuring capacitor and the meter. The level of this transient voltage is controlled by the Zener diode on the input side of the capacitor. The Zener-limited transient voltage causes the measuring capacitor to be rapidly charged to the regulated voltage, with the charging current flowing through the series circuit with the meter. When the ignition capacitor again recharges, the measuring capacitor empties or discharges to a zero charge level as a result of the similar grounded or reference diodes to the opposite side thereof. Thus, each discharging of the ignition capacitor results in a precise filling of the measuring or "bucket" capacitor with a given electrical charge. As a result, a corresponding current flow occurs in the meter which functions to indicate the average current flow which is proportional to the speed of the engine.
Various available capacitor discharge ignition systems employ a single capacitor which is charged to a voltage level of the appropriate polarity while others employ a pair of capacitors which are alternately charged and discharged. Further, depending upon the particular circuit design, the operating voltage levels may be different. For example, in one circuit design a single capacitor is sequentially charged to a level of 100 to 150 volts and then discharged; the capacitor may be charged two or three or four times per revolution depending on whether connected to a two or three or four-cylinder engine. In more recent versions, a single capacitor is charged to a level of 200 to 300 volts and discharged an appropriate number of times per crankshaft revolution. In one multiple capacitor design, particularly applied to a two cylinder, two cycle engine, one capacitor is charged to a positive voltage level of from 200 to 300 volts and the second capacitor is charged to a corresponding negative voltage. The capacitors are alternately charged and discharged, each being discharged once per revolution.
A suitably-calibrated tachometer circuit may be employed with any of the aforementioned capacitive discharge ignition systems as each exhibits the desired steep wave front when the capacitor discharges and a capacitor is discharged at least once per revolution.
Generally, the ignition circuits are potted in a suitable protective resin with appropriate external terminals or connections for connecting to an alternator power source, trigger pulse generator, output pulse transformers, and an ignition shut-off switch. In certain systems an external connection to the ignition capacitor means is not available. In such a system, a special adaptor must be interconnected to some other portion of the ignition circuit to duplicate externally of the ignition box the voltage wave shape of the capacitor means.
The prior art tachometer circuits incorporating a measuring capacitor are used commercially, but although appearing quite simple are relatively expensive and difficult to produce to a high degree of accuracy and stability. Generally, the prior art tachometer circuits are temperature sensitive and the degree of sensitivity of different circuits of the same design may vary. For example, the effective size of the measuring capacitor and the level of filling of the capacitor varies from circuit to circuit with temperature as hereinafter discussed. Ignition capacitor voltage level variations may occur over the speed range for any given ignition system. The ignition capacitor voltage levels may vary from engine to engine even on supposedly identical engines. These variations, in turn, will result in small errors in the readings of the tachometers. Although the accuracy variation is not such as to create a hazardous condition, such tachometers do not provide a highly accurate indication which may be significant in obtaining optimum engine performance particularly in the operation of highly tuned engines and the like.
In addition, a very significant cost factor in the commercial production of tachometers arises from the relatively high inventory costs and relatively expensive calibration procedures required to produce a consistently accurate tachometer.
For example, circuit designers recognize that the measuring capacitor, selected because of practical considerations, has a positive temperature coefficient such that the electrical capacitance of the capacitor increases somewhat with increasing temperature. Compensation is conventionally provided by connecting a relatively small compensating capacitor, having a large negative temperature coefficient, in parallel with the measuring capacitor. The compensating capacitance desirably is small for reasons of economy and overall temperature stability, inasmuch as the temperature characteristic of the compensating capacitor is difficult to control precisely. This, however, requires the charging of the compensating and measuring capacities to a relatively high voltage and, consequently, requires the use of a high voltage Zener diode for limiting of the charge level to a selected value during each cycle. High voltage Zener diodes, however, generally have a significant positive temperature coefficient and this requires further temperature compensation. Although temperature stable diodes are available, the cost is prohibitive. As a practical matter, commercial design is therefore a compromise employing a relatively smaller measuring capacitor and a plurality of relatively small, but somewhat less than optimum sized, compensation capacitors connected in parallel. The combination overcorrects for the temperature coefficient of the measuring capacitor in an attempt to at least partially compensate or cancel the effects of the positive temperature coefficient of the high voltage Zener diode.
Such design has provided at best, a partial alleviation of the circuit's temperature compensation. This is particularly true where the small compensating capacitors are also, and primarily, employed as the calibration elements. Thus, the circuit components employed must include manufacturing tolerances and each tachometer circuit must, therefore, include some individually selected component to achieve the required adjustment for the tachometer calibration in combination with the actual values of other components. In practice, the compensation capacitors which have to be wired into the circuit have provided a convenient component and, therefore, the designers have accepted a semi-optimum temperature compensation in favor of this method of calibration.
Generally, the tachometer unit is assembled to the point of connecting a pair of calibrating semi-compensating capacitors in the circuit. A selector switch unit is first connected to the partly assembled module and connects various selectable values of available calibrating capacitors into the tachometer circuit. The individual performing the calibration reads the position of the selector switch unit when a proper output reading is obtained and is thereby informed of the proper selection from the set of available capacitors. The measured sizes of calibration capacitors are taken out of stock and then soldered into the module, normally by hand. The calibrating capacitors are now normally individually checked.
The use of a pair of calibrating capacitors requires a very significant inventory of capacitors because a plurality of relatively small capacitance values is required in order to properly provide calibration plus partial temperature compensation for the normal manufacturing tolerances encountered in commercially available Zener diodes and the relatively large measuring capacitors. Further, unless large quantities are purchased with each order, the individual cost of the calibrating capacitors is prohibitively high.
Further, in general production practice, a linear precision resistor is used as the load element during the calibration procedure. The average current is measured as a DC voltage appearing across this precision resistor. Although this theoretically is satisfactory, the measurement in fact ignores the effects of the inductance in the standard microammeter.
In practice, the microammeter generally includes a coil positioning a readout pointer, and the coil introduces inductance in the charging circuit of the measuring capacitor. Such inductance tends to establish a freewheeling current effect and maintains current flow through the meter in excess of the charge flow into the measuring capacitor. This, of course, further contributes to slight inaccuracy in the RPM reading of the tachometer.
The magnitude of the free-wheeling phenomena also changes with the operating voltage level of the ignition capacitor and introduces a variable source of error in the tachometer circuit.
Further, with the various systems employing different discharges per revolution of the ignition capacitors, the size of the measuring capacitor must be adjusted accordingly to produce the same meter deflection with the measuring capacitor charged to a common level. This, of course, is desirable since it allows standardization upon a single basic RPM calibrated DC meter. The average current flow varies with the number of charges per revolution which may be two charges per revolution for one system and four charges per revolution for a second system. The size of the measuring capacitor for the first system must be twice that of the second system, if the same meter deflection is to be developed at the same speed. Two different circuit modules are designed for use with the single standardized meter. This further contributes to the total cost of inventory and increases the need for tachometer apparatus which can be constructed with reduced costs.